1. Field of the Invention
The present invention relates to a display device including R, G, B, and W sub-pixels, and more particularly, to determining a use rate of the W sub-pixel.
2. Description of the Related Art
FIG. 1 illustrates an example of a typical dot arrangement of a matrix type organic electroluminescence (EL) panel 1 in which three sub-pixels 3 of red (R), green (G), and blue (B) constitute one pixel 2. FIGS. 2 and 3 illustrate examples of a dot arrangement of a matrix type organic EL panel 1 which uses sub-pixels 3 including white (W) in addition to R, G, and B. In FIG. 2, sub-pixels 3 are arranged in a horizontal direction to form one pixel 2. In FIG. 3, sub-pixels 3 are arranged in 2×2 matrix to form one pixel 2. The RGBW type organic EL panel 1 uses the W sub-pixel, which is higher in emission efficiency than R, G, and B sub-pixels, in order to reduce power consumption and increase luminance of the panel. Methods for realizing the RGBW type panel include a method involving using organic EL elements emitting respective colors of the sub-pixels, and a method involving overlaying red, green, and blue optical filters on white organic EL elements to realize the sub-pixels other than the W sub-pixel.
FIG. 4 is a CIE 1931 chromaticity diagram illustrating an example of a chromaticity of white (W) used for the white pixel in addition to the three typical primary colors of red (R), green (G), and blue (B). Note that, the chromaticity of W does not necessarily need to match reference white of the display.
FIG. 5 illustrates a method of converting R, G, and B input signals, with which the reference white of the display may be displayed when R=1, G=1, and B=1, to R, G, B, and W image signals. First, if the emission color of the W sub-pixel does not match the reference white of the display, the following operation is performed on the input RGB signals for normalization to the emission color of the W sub-pixel.
                    Equation        ⁢                                  ⁢        1                                                                      [                                                    Rn                                                                    Gn                                                                    Bn                                              ]                =                              [                                                            a                                                  0                                                  0                                                                              0                                                  b                                                  0                                                                              0                                                  0                                                  c                                                      ]                    ×                      [                                                            R                                                                              G                                                                              B                                                      ]                                              [                  Num          .                                          ⁢          1                ]            where R, G, and B are input signals, Rn, Gn, and Bn are normalized red, green, and blue signals, and a, b, and c are coefficients selected so that a luminance and a chromaticity equivalent to those obtained when W=1 are obtained when R=1/a, G=1/b, and B=1/c.
Examples of the most fundamental operational expressions for S, F2, and F3 include:S=min(Rn,Gn,Bn)  Equation 2F2(S)=−S  Equation 3F3(S)=S  Equation 4
In this case, as the color of the displayed pixel is more achromatic, the ratio at which the W sub-pixel is lit increases. Therefore, as the ratio of near-achromatic colors in a displayed image increases, the power consumption of the panel becomes lower as compared to the case where only R, G, and B sub-pixels are used.
The final normalization to the reference white is, similarly to the normalization to the emission color of the W sub-pixel, processing performed when the emission color of the W sub-pixel does not match the reference white of the display, and involves performing the following operation:
                    Equation        ⁢                                  ⁢        5                                                                      [                                                                      R                  ′                                                                                                      G                  ′                                                                                                      B                  ′                                                              ]                =                              [                                                                                1                    /                    a                                                                    0                                                  0                                                                              0                                                                      1                    /                    b                                                                    0                                                                              0                                                  0                                                                      1                    /                    c                                                                        ]                    ×                      [                                                                                Rn                    ′                                                                                                                    Gn                    ′                                                                                                                    Bn                    ′                                                                        ]                                              [                  Num          .                                          ⁢          2                ]            
In general, few images are constituted only of pure colors, and the W sub-pixel is used in most cases. Therefore, the overall power consumption is reduced in average as compared to the case where only R, G, and B sub-pixels are used.
In a case where the following equations are used for F2 and F3, the use rate of the W sub-pixel changes depending on the value of M.F2(S)=−MS  Equation 6F3(S)=MS  Equation 7where M is a constant in the range of
In view of power consumption, it is most preferred to use M=1 as expressed by Equations 2 to 4, that is, the use rate of 100%. However, in view of visual resolution, it is preferred to select such a value of M that all the R, G, B, and W sub-pixels are lit where possible. This is described below in detail.
In a panel in which R, G, and B sub-pixels are arranged in matrix as illustrated in FIG. 1, in order to improve the visual resolution, a phase of a signal of each color and the position of each sub-pixel in the panel are aligned as illustrated in FIG. 6. In this case, the resolution in the horizontal direction of the input image signal is three times the number of horizontal pixels of the panel, and hence needs to be the same as the number of sub-pixels in the horizontal direction of the panel. However, when sampling is performed for each color at timings as illustrated in FIG. 6, the apparent resolution increases. In other words, when the phase of each color signal and the position of each sub-pixel are aligned, it is possible to obtain a display image which is higher in apparent resolution than that obtained in a case where three sub-pixels of R, G, and B are all driven with signal data of the same phase (FIG. 7). This is because luminance information at the position of each sub-pixel of the input signal may be reproduced to some extent by the luminance component of each color.
Also in the case where R, G, B, and W sub-pixels are used as illustrated in FIGS. 2 and 3, it is possible to increase the apparent resolution in a similar manner by aligning the phase of each color signal and the position of each sub-pixel of the panel. FIG. 8 illustrates an example of sampling when the use rate of the W sub-pixel is approximately 50% in the case where the sub-pixels are arranged as illustrated in FIG. 2.
When the use rate of the W sub-pixel is 100%, that is, in a case where M=1 in Equations 6 and 7, as the image becomes more achromatic, the effect becomes lower because the amount of light emitted by the R, G, and B sub-pixels becomes smaller. In particular, in the case where the primary W color is identical to the reference white, R, G, and B sub-pixels are not used at all when a black and white image is displayed, with the result that the resolution is the same as the number of W sub-pixels as illustrated in FIG. 9.
As described above, the power consumption and the apparent resolution change depending on the value of M and are in a trade-off relation. Therefore, in Japanese Patent Application Laid-open No. 2006-003475, spatial frequency components of a displayed image are partially detected, and the use rate (M) of the W sub-pixel is adaptively changed depending on the detection result, to thereby suppress reduction in resolution and reduce power consumption.
According to the method of Japanese Patent Application Laid-open No. 2006-003475, an average power consumption considerably close to that obtained when M=1 may be obtained depending on the picture, and at the same time, the value of M may be decreased at edge portions of the image to improve the image quality. However, even with this method, the image quality at portions that are near-achromatic and low in spatial frequency may appear poorer than that in a case where M is constant and optimized for the image quality. Specifically, M becomes large at the portions so that only the W sub-pixels are brightly lit to be seen as stripes in the case of the arrangement of FIG. 2 and as dots in the case of the arrangement of FIG. 3 from a close distance.
A person with a visual acuity of 1.0 has a resolution of 1 arc minute of visual angle, and it is said that, if the number of scanning lines is 1,100, the scanning lines are invisible when the viewing distance is 3H (3 times the height of screen) or more. Therefore, when viewed from a predetermined distance or more, there is no problem in image quality even when M=1 in the case of the display device including square pixels as illustrated in FIGS. 2 and 3. As described above, in the case of the display device in which one pixel is constituted of a plurality of sub-pixels, it is desired that the image be viewed from such a distance that each sub-pixel cannot be distinguished. However, it is difficult to always satisfy this condition, because the size of the sub-pixel varies depending on specifications of the number of pixels, the screen size, and the like, and because the distance between the display device and the viewer changes depending on the use environment.
Further, in applications such as digital signage, there may be cases where, although people are usually located away from the display device, one of the people who felt interest in the content of the digital signage may approach the digital signage to take a close look at the content.